In conventional crankcase scavenged two-stroke-cycle engines (two cycle engines) with piston controlled intake and exhaust ports, it is generally desired to minimize crankcase volume (CV). This maximizes the pressure ratio (PR) from the crankcase compression process. Pressure ratio (PR) equals the peak crankcase pressure divided by ambient pressure.
The relationship in a particular engine between PR and CV normalized by displacement (D) at one engine speed is shown in FIG. 1. A representative value of CV/D is also shown for reference. FIG. 2 shows how the crankcase pressure varies with crank angle for several engine speeds at the reference CV/D. Note that the peak pressure increases with speed as does the crank angle duration from peak pressure to ambient pressure.
An important design goal is to maximize engine power at the rated speed. Such a result follows from optimizing the scavenging process; however, if scavenging is optimized at rated speed, it will not be optimum at lower speeds unless the fluid mechanical process can be made similar. Such similarity requires that the ratio of fresh charge jet velocity to mean piston speed (JV/PV) be relatively constant and the duration of the blowdown process in crank angle degrees should not vary with speed.
FIG. 3 shows a representative variation of the ratio JV/PV over a speed range of interest and the corresponding variation of blowdown duration (BD) in crank angle degrees. As the engine speed is reduced from a maximum, the increase in JV/PV suggests relatively more rapid mixing of the jetting fresh charge with the expected result of a lower mass of trapped fresh charge. Decreasing the crank angle duration for blowdown as the engine speed is reduced suggests that a larger fraction of the fresh charge would enter at a smaller inlet port open area and this would very likely degrade the quality of the scavenging process.